subscriber: add flatten span option to json formatter

[heat1q]

Motivation

Span fields are currently not accessible at root level but only inside a nested object or the span list. This makes indexing log data more complex if a unique span identifier is required at root level, for example in the case of distributed trace Id (See also #1531).

Solution

This change adds two new options to Json for making span fields accessible at root level:

• Json::flatten_current_span flattens all span fields into the root object. The name field is not carried over, since it might be ambiguous at root level.
• Json::flatten_span_list flattens all fields for each span in the list into the root object. Colliding fields will be overwritten from root to leaf span.

Examples

Default

{
  "timestamp": "2023-08-30T13:28:57.999070Z",
  "level": "INFO",
  "fields": {
    "message": "shaving yaks"
  },
  "target": "fmt_json::yak_shave",
  "span": {
    "yaks": 3,
    "name": "shaving_yaks"
  },
  "spans": [
    {
      "yaks": 3,
      "name": "shaving_yaks"
    }
  ]
}

Flatten only current span

All fields (except name) of the current span are present at the root level.

tracing_subscriber::fmt()
    .json()
    .flatten_current_span(true)
    .flatten_span_list(false)
    .init();

{
  "timestamp": "2023-08-30T13:30:22.753644Z",
  "level": "INFO",
  "fields": {
    "message": "shaving yaks"
  },
  "target": "fmt_json::yak_shave",
  "yaks": 3,
  "spans": [
    {
      "yaks": 3,
      "name": "shaving_yaks"
    }
  ]
}

Flatten current span and span list

tracing_subscriber::fmt()
    .json()
    .flatten_current_span(true)
    .flatten_span_list(true)
    .init();

{
  "timestamp": "2023-08-30T13:31:17.733524Z",
  "level": "INFO",
  "fields": {
    "message": "shaving yaks"
  },
  "target": "fmt_json::yak_shave",
  "yaks": 3
}

Resolves: #2670

[msabansal]

Is there any followup needed on this pr? Can we get this reviewed please.

[heat1q]

Is there any followup needed on this pr? Can we get this reviewed please.

Nothing from my side. We need to wait for the maintainers to take a look.

[coltfred]

I'd love to see this added as well. We're using spans to track ray_ids for our HTTP requests and want to be able to get rid of the span name and flatten the ray_id onto the top object.

[Turbo87]

@hawkw @davidbarsky @hds any chance one of you could take a look? we would like to use this for crates.io too :)

[mladedav]

For anyone interested, I've created a json-subscriber and I'm trying to implement all these long standing feature requests there (or making it general enough that people can set most things themselves). I've added flattening of spans there too.

[dspatoulas]

Thank you @heat1q! This is exactly what I was hoping to find after enabling flatten_events and realizing that's only half the battle.